There are known methods for forming products from semi-molten metal as methods of producing high quality products. The following methods for using the semi-molten melt are adopted:
Semi-melting forging method is to forge material in the semi-melting state between forging molds or dies to shape and cool a product.
Semi-melting casting method is a method of casting by injecting a semi-molten metal from a sleeve of a high pressure casting machine. In the process the semi-molten melt is prepared previously in some furnace from a billet as a material, carried to the sleeve, and injected from the sleeve into the casting mold.
Semi-melting injection molding method is known as a method of injecting into a mold a semi-molten metal which is prepared in the desirable semi-molten state inside a injector and pressurized by the very injector to mold a product with desired shape in a cavity of the mold. In this method, the melt is prepared to be in a semi-melting state (i.e. in a state mixing of a liquid phase and a solid phase in a metal or alloy) inside the injector by heating and melting the powdered or pelletized metal material in the temperature-controlling cylinder of the injector. The melt is pressurized by the screw toward a nozzle attached to an end of the cylinder and injected into a cavity inside the mold in connection to the nozzle.
This semi-melting injecting molding method has been provided aluminum and magnesium metal/alloy products with high quality and less defect.
In the prior art regarding the semi-melting casting, Japanese Patent Publication No 7-256427 A discloses a method of pouring the melt in the semi-molten metal in a sleeve which is provided close to and passing through the cavity and injecting the melt pressurized by a plunger from the sleeve into the cavity. In the prior art the method can pressurize the melt in the cavity partially by the plunger to overflow into a basin past the cavity through so narrow pass as to filtrate grains of the solid phase in the melt. Thus, the remaining melt in the cavity by filtration has partially higher solid fraction than one in another portion of the cavity. This process can control to differ the solid fractions in some portions of the molded product.
Some cast or molded products such as mechanical components are often required to be provided with different properties in their different portions. Such properties are dimensional accuracy, mechanical property including tensile strength of the product, and the like.
To meet the request there have been used the way of setting the solid fraction of each portion to desired different value, the different solid fractions changing the metal micro-structure of each portion in the metal product after solidified.
Solid fraction of melt has been found to have a relation with shrinkage of cast during solidifying of the metal. FIG. 5A shows that a magnesium alloy cast decreases in dimension change due to shrinkage as the solid fraction of the alloy melt increases and that high solid fraction is required in a portion in which high molding dimensional accuracy is requested for the product. In general, the semi-molten melt contains a solid phase as grains which are dispersed in the liquid metal. As the melt is poured into a mold, the liquid part of the melt supplied is solidified in a cavity of the mold, and the solid fraction formed previously can reduce the shrinkage of the metal product because solid phase dose not almost reduce in volume during solidification. Thus the semi-melting injection method allows the product to obtain high dimensional accuracy.
Solid fraction of the semi-molten melt also have a relation with ultimate tensile strength of the product. FIG. 5B shows that a magnesium alloy product tends to decrease in as-cast tensile strength as the solid fraction in the alloy melt increase. It can be seen that a low solid fraction in the melt is needed to enhance the mechanical property in the strengthening portion of the cast product.
Thus, in the methods employing the semi-molten metal, the solid fraction should be set in each portion in a product in a particular, different value according to the required property for each portion of the product.
However, the prior-art method mentioned above has a disadvantage as follows; the method of using the pressurizing the melt locally in the cavity can manage to vary the solid fractions between the flow range of the pass to the partial pressurized portion and other portion in the cavity, but the locally pressurizing a melt outside the cavity is attended with difficulty of controlling the desired solid fraction in a wide range quantitatively in the desirable portion of the product, and also reduction in yield of production due to the plunger pushing out a part of the melt. The mechanism is also much complicated by arrangement of the sleeve and the plunger movable in it near the cavity of the mold.